Rebalance table shards behavior clarification

http://docs.citusdata.com/en/v7.5/develop/api_udf.html#rebalance-table-shards

This should make it more clear that, when you choose a table to rebalance, all co-located tables also get rebalanced with it.

## Why are we implementing it? (sales eng)

Existing phrasing implies that steps needed are significantly more complicated than necessary. End user confusion could result in issues because of this.
This blog may help provide relevant phrasing:
https://www.citusdata.com/blog/2017/04/11/rebalancing-your-database-with-citus/